I. Field
The present document relates generally to wireless communication and amongst other things to eigen-beam forming for wireless communication systems.
II. Background
An orthogonal frequency division multiple access (OFDMA) system utilizes orthogonal frequency division multiplexing (OFDM). OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple (N) orthogonal frequency subcarriers. These subcarriers may also be called tones, bins, and frequency channels. Each subcarrier is associated with a respective sub carrier that may be modulated with data. Up to N modulation symbols may be sent on the N total subcarriers in each OFDM symbol period. These modulation symbols are converted to the time-domain with an N-point inverse fast Fourier transform (IFFT) to generate a transformed symbol that contains N time-domain chips or samples.
In a frequency hopping communication system, data is transmitted on different frequency subcarriers during different time intervals, which may be referred to as “hop periods.” These frequency subcarriers may be provided by orthogonal frequency division multiplexing, other multi-carrier modulation techniques, or some other constructs. With frequency hopping, the data transmission hops from subcarrier to subcarrier in a pseudo-random manner. This hopping provides frequency diversity and allows the data transmission to better withstand deleterious path effects such as narrow-band interference, jamming, fading, and so on.
A problem in most communication systems is that the receiver is located in a specific portion of an area served by the access point. In such cases, where a transmitter has multiple transmit antennas, the signals provided from each antenna need not be combined to provide maximum power at the receiver. In these cases, there may be problems with decoding of the signals received at the receiver. One way to deal with these problems is by utilizing beamforming.
Beamforming is a spatial processing technique that improves the signal-to-noise ratio of a wireless link with multiple antennas. Typically, beamforming may be used at either the transmitter or the receiver in a multiple antenna system. Beamforming provides many advantages in improving signal-to-noise ratios which improves decoding of the signals by the receiver.
Certain types of OFDMA systems are frequency division duplexed (FDD) OFDMA systems. In these FDD OFDMA systems, the transmission from the access point to the access terminal and from the access terminal to the access point occupy different distinct frequency bands. In FDD OFDMA systems feedback to perform beamforming generally requires knowledge of the channel at the transmitter, e.g. access point, which is not available without substantial feedback. This feedback, generally in the form of the actual beamforming weights or vectors, requires a large amount of resources on control or signaling channels. This reduces data rates and increases the overhead required.
Therefore, it is desired that systems provide feedback for more accurate beamforming while minimizing the resources needed to provide the feedback from the receiver to the transmitter.